Wolf-Rayet Stars
For a classification of stars see The H-R Diagram. Wolf - Rayet are a special type of stars. They are very hot, depleted of hydrogen and develop very strong stellar winds. In these stars, photon radiation pressure is higher then gravity. As a result, matter is pushed away from the star. The star There are two different types of WR stars, depending on mass. Large WR stars are evolved O - type stars. As they start fusing helium or heavier elements, they produce more energy, increasing their diameter. Smaller stars would become Red giants. As O - type stars increase their size, gravity on their outer layers of gas is too small and radiation pressure overpasses gravity, expelling matter into space. These stars have convective currents, mixing matter between the core and the outer layers. This way, we can see what the star is fusing and how much time it has left. Massive stars fuse helium fast (and heavier element even faster). The rate at which they lose matter is too low to prevent them from going supernova. Small WR stars are the last phase of small red giants. Too small to go supernova, these stars fuse helium. It is the helium flare that gently expels the outer hydrogen envelope into space, letting the inert and compressed carbon-oxygen core in the end to be exposed and become a White Dwarf. Environment around a WR star These stars lose matter at an alarming rate, creating a very powerful solar wind. This creates a nebula surrounding them. For any existing planet, the solar wind will be a challenge. Unlike red giants, WR stars create winds that are strongly ionized. In case of large WR stars, the wind usually consists of helium, nitrogen, carbon, oxygen or even silicon (showing what the star is fusing in its core). In rare cases, the wind contains hydrogen. In case of small WR stars, the wind consists only of hydrogen and helium. Strong solar winds can erode atmospheres around smaller planets, but also might feed giant planets and make them grow. Rocky planets without strong magnetic fields will lose their atmospheres. Also, around gas giants, moons might not be well protected by their planets' magnetic fields, which might not resist the incoming winds. WR stars are hot (30000 to 200000 K), so their radiation contains high doses of far UV and X rays. This is a great hazard for life. Also, ionizing radiation can split water into hydrogen and oxygen, depleting planets of it. Evolution of WR stars WR stars have a short lifetime, comparable to the life of a human civilization. One they finished fusing helium, there is only little time until they go supernova. In all cases, the mass loss will influence orbits of any planets, which will move outwards. So, a planet will be for a limited time in the Habitable Zone where it once were when the star was more stable. See Wolf-Rayet Stars Approaching Supernova for more details. Planets and terraforming Probably, rocky planets around WR stars are depleted of water and atmospheres. However, given the short time of the WR phase, there might still be enough water, especially in case of a former Outer Planet or an Oceanic Planet. Scientists must analyze if a terraformed planet will support its atmosphere for long enough and if the star will remain as it is for enough time. Human civilizations have a short lifetime. If, however, the planet cannot be maintained habitable for at least 1000 years, terraforming should be out of question. In case of large WR stars, since they end-up with a supernova, a terraformed planet might end-up with a violent death. Even if the planet might survive, the surface will be heated to extreme temperatures and the explosion will destroy any structure on the surface. In case of small WR stars, which end-up as White dwarfs, after the WR step, the planets will receive very little heat and light. Even with high amounts of Greenhouse Gases, there might be too little Luminosity for plants to survive, assuming we can maintain a temperature suitable for Earth life forms. Still, human colonies can survive. Even if terraforming is not possible or too expensive, limited Paraterraforming might be feasible. Industrial colonization could bring settlers to the planets, moons and asteroids surrounding a WR star. Category:Stars and other hosting celestial bodies